1. Field of the Present Invention
The present invention relates to a positive electrode active material and a non-aqueous electrolyte lithium ion secondary battery using thereof. More particularly, the present invention relates to an active material for the positive electrode of a non-aqueous electrolyte lithium ion secondary battery, which is comprised of a spinel lithium manganese composite oxide having a large discharge capacity and a secondary battery using that material.
2. Description of the Related Art
A lithium secondary battery and a lithium ion secondary battery (hereinafter called “lithium based secondary battery”) are characterized by having a large capacity for its small size, and are widely used as a power supply for cell telephones, notebook type personal computers and so forth. While LiCoO2 is mainly used as an active material for a positive electrode (hereinafter referred to as “positive active material”) of a lithium based secondary battery at present, safety in its charge state is not particularly sufficient and the price for the Co material is high. In this respect, active studies are being made on new positive active materials which replace LiCoO2.
Consideration is being made on the use of LiNiO2 as the material which has a layer-like crystal structure similar to that of LiCoO2. LiNiO2, which demonstrates a high capacity, has a lower potential than LiCoO2 and still has a problem on safety in charging.
Active studies are also being made on the use of LiMn2O4 with a spinel structure as another positive active material. However, LiMn2O4 suffers cycle-dependent degradation and a reduction in capacity at high temperature. Those appear to be originated from the instability of trivalent Mn and the cycle-dependent deterioration of the performance or the like seems to occur because Jahn-Teller distortion occurs in the crystal at the time the mean valence number of Mn ions changes between trivalence and tetravalence.
In view of the above, studies have been made to improve the structural stability by replacing trivalent Mn with other elements in order to enhance the reliability of the battery. For example, Japanese Patent Laid-Open No. 2001-176557 discloses a secondary battery having such a positive active material and discloses an active material in which trivalent Mn included in LiMn2O4 is replaced with other metals. Specifically, the claims in the publication describe a secondary battery having a lithium manganese composite oxide, which has a spinel structure and is expressed by a composition formula of LiMxMn2−xO4 (where M represents one or more types of elements selected from Al, B, Cr, Co, Ni, Ti, Fe, Mg, Ba, Zn, Ge and Nb and 0.01≦x≦1). The detailed description of the present invention in the publication specifically discloses an example which uses LiMn1.75Al0.25O4 as a positive active material.
In case where trivalent Mn is reduced by substitution of another element as mentioned above, however, a reduction in discharge capacity should be coped with. As charge and discharge take place, the valence number of Mn in LiMn2O4 changes as follows.Li+Mn3+Mn4+O2−4→Li++Mn4+2O2−4+e−As apparent from this formula, LiMn2O4 contains trivalent Mn and tetravalent Mn and brings about discharging as the trivalent Mn is changed to tetravalent Mn. Substituting another element for the trivalent Mn, therefore, inevitably decreases the discharge capacity. That is, while the reliability of the battery is improved by increasing the structural stability of the positive active material, a reduction in discharge capacity becomes prominent. It is therefore difficult to satisfy both. Particularly, it is extremely difficult to acquire a highly-reliable positive active material with a discharge capacity of 130 mAh/g or greater.
As apparent from the above, an active material in which trivalent Mn included in LiMn2O4 is replaced with another metal constitutes a lithium secondary battery having so-called 4-V class electromotive force. Techniques in different aspects have also been studied. In Japanese Patent Laid-Open No. 147867/1997, for example, Ni, Co, Fe, Cu, Cr or the like is substituted for part of Mn in LiMn2O4 to increase the charge/discharge potential, thereby increasing the energy density. Those techniques construct a lithium secondary battery having so-called 5-V class electromotive force. A description of this type of lithium secondary battery will be given below of a specific example of LiNi0.5Mn1.5O4.
LiNi0.5Mn1.5O4 changes the valence number of Ni in accordance with charging and discharging.Li+Ni2+0.5Mn4+1.5O2−4→Li++Ni4+0.5Mn4+1.5O2−4+e−
As apparent from this formula, LiNi0.5Mn1.5O4 causes discharging as bivalent Ni changes to tetravalent Ni. Mn does not change its valence number. Electromotive force of 4.5 V or greater can apparently be acquired by changing metal associated with charging and discharging to Ni or Co or the like from Mn.
Japanese Patent Laid-Open No. 2000-235857 discloses a crystal LiMn2−y−zNiyMzO4 (where M denotes at least one selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu, 0.25≦y≦0.6 and 0≦z≦0.1) with a spinel structure, which performs charging and discharging with respect to Li at a potential of 4.5 V or higher. Japanese Patent Laid-Open No. 2002-63900 discloses a 5-V class positive active material in which Mn in LiMn2O4 is replaced with another transition metal and is further replaced with other elements and which is expressed by a general formula LiaMn2−y−i−j−kMyM1iM2jM3kO4 (where M1 is a bivalent cation, M2 is a trivalent cation, M3 is a tetravalent cation, M is an at least one type of transition metal element excluding Mn, i≧0, j≧0, k≧0 and i+j>0).
Even the use of such an active material however still makes it difficult, at present, to significantly surpass LiCoO2, currently available, in energy density. While the 5-V class active materials can indeed generate electromotive force of 4.5 V or greater, however, they have a problem that the discharge capacity is reduced.